The invention relates to a vertical bipolar transistor, and particularly to a vertical bipolar transistor suitable for use at high frequencies.
Bipolar transistors are used in a number of applications, including in high-voltage radio frequency devices. The design of such structures is a trade-off between a number of factors. One factor is the breakdown voltage between the collector and the base or emitter, i.e. the maximum voltage that may be applied between the collector and the base or emitter when applying a reverse potential to the collector without causing breakdown. Another factor is the cutoff frequency of operation. It would be desirable to increase the values is of both of these parameters.
However, it is well known in the art that in general the product of the cutoff frequency ft and the breakdown voltage between the collector and the emitter has a maximum known as the Johnson limit. This product is accordingly an important parameter for bipolar transistors. Since the product has a maximum, it is not normally possible to increase one of these parameters without reducing the other.
An exception is known to the Johnson limit at very high frequencies. Non-local avalanche effects in the base-collector space charge region can allow the Johnson limit to be exceeded at the very highest radio frequencies.
However, it would be useful to be able to design transistors for which the value of the threshold frequency and breakdown voltage product exceeds the Johnson limit over a broad radio frequency range.
According to the invention there is provided a bipolar transistor structure, comprising: a collector including a higher doped collector region of semiconductor material of a first conductivity type doped to a first concentration; an emitter region of semiconductor material of the first conductivity type; a base region of semiconductor material of a second conductivity type opposite to the first conductivity type between the emitter region and the collector; the collector further including a lower doped drift region extending between the higher doped collector region and the base region, the drift region being of the first conductivity type and doped to a second concentration lower than the first concentration; a trench extending adjacent to the drift region; and a gate within the trench insulated from the drift region for controlling the drift region to be depleted of carriers in a voltage blocking mode of operation.
The drift region in the collector is of lower doping concentration than the higher doped region of the collector so that the drift region may be depleted of carriers. Using the gate in the trench the drift region can be depleted even with a higher doping in the drift region than would otherwise be possible. This allows the product of the threshold frequency and the breakdown voltage to be increased as compared with prior art structures. In embodiments of the invention, the Johnson limit may be exceeded.
Conveniently, the structure may be a vertical structure formed on a semiconductor body having opposed first and second faces. The emitter region may be connected to the first face and the collector region to the second face. The trench may extend substantially perpendicularly to the first face through the emitter and base regions to the drift region.
In alternative embodiments, a lateral structure may be provided, for example using an insulated buried layer as the gate.
The gate may be separated from the drift region by a gate insulating layer on the sidewalls of the trench.
The structure is also typically easier to manufacture than structures involving multiple layers in the drift region.
The collector region may include a semiconductor substrate or body or a layer or region formed on a substrate. The trench may extend through the emitter, base and drift regions.
The invention is of particular application to high frequency devices. Such devices, may be for example heterostructure bipolar transistors. The invention can also be used for low frequency devices.
The gate may be of a semi-insulating material, and the structure may further comprise a first gate connection at the end of the gate adjacent to the boundary between the drift region and the base region and a second gate connection at the boundary between the drift region and the higher doped collector region. This allows a uniform field to be applied along the gate thereby providing a uniform field in the drift region to minimises the risk of breakdown at low voltages. The uniform field is achieved without complex doping profiles in the drift region being necessary.
The contact at the end adjacent to the base may be electrically connected to the emitter (or base). Thus, the first gate connection may be a gate contact in electrical connection with a base contact contacting the base or an emitter contact contacting the emitter. In alternative embodiments, there may be a separate connection to the gate to allow the voltage across the gate to be controlled independently of the voltage on the emitter.
The second gate connection may be a direct connection between the end of the drift control gate adjacent to the higher doped region of the collector and the higher doped region of the collector. Alternatively, the second gate connection may be a further contact so that the voltage applied along the gate can be controlled independently.
Alternative embodiments replace the semi-insulating gate with a p-i-n diode having voltage dropped across the intrinsic (i) region to very similar effect.
Alternatively, the gate may be conducting and a uniform field in the drift region may be provided by a suitable graded doping profile in the drift region. In this particular embodiment the gate is isolated from the highly doped collector region.
Other advantageous technical features of the present invention are set out in the attached dependent claims.
In another aspect the invention also relates to a method of manufacturing the bipolar transistor described above.